Sounds from the ground: geologists dig into the heart of the mystery known as the 'Moodus noises.' (includes related article)

Sounds From the Ground

When sudden booms or thunderous rumbles echo off the hills of Moodus, the residents of this small south-central Connecticut town seldom dive for cover. It's just another one of the "Moodus noises" -- created by the small, shallow earthquakes that have frequented the area for at least 300 years and perhaps much longer. But while modern Moodus residents may be more or less unimpressed with these sounds from beneath, the quakes that create the noises have captured the attention of earth scientists from around the United States.

Since 1979, when Boston College's Weston Observatory established a network of seismometers near Moodus, this area has hosted four swarms of dishrattling earthqueakes that peaket at about magnitude 2.5 and lasted up to several months long. Each swarm consisted of hundreds of tiny earthquakes, all originating from a small spot in the earth's crust near the north end of town.

"The mystery is: Why are these earthquakes occurring there?" says seismologist John Ebel of the Weston Observatory. "Most areas in the eastern United States that we watch seismically donht seem to have such a persistent earthquake activity centered in one very small locality as Moodus does."

The quest to understand the Moodus quakes has drawn together researchers from any branches of the earth sciences, who met to discuss their work at a recent all-day symposium of the American Geophysical Union in Baltimore. Studying the area from the surface, geologists have mapped out the faults and folds that tell the history of the earth's crust around Moodus. Seismologists have peered into the subsurface through measurements of the ground's motion during earthquakes.

The Moodus quakes have also interested companies that run electric power plants in New York and Connecticut -- including one nuclear facility located i n Haddam, Conn., only 20 miles from Moodus. In an effort to assess the potential for large earthquakes in the area, these utilities funded the drilling last year of a 1.5-kilometer-deep hole close to the earthquake epicenters.

Yet, despite of all the information they have gathered, scientists remain puzzled. Says Ebel, "I think the sum total of all the evidence is that it's not entirely clear why the earthquakes are occurring there, why there aren't other areas like this a few miles away."

Experiments in the borehole are revealing the crustal forces beneath Moodus related to the earthquakes, and in the process they are correcting some earlier ideas about the geologic stress in this area. Earlier, less reliable studies in shallow boreholes had suggested that the crust of Moddus different markedly from most areas east of the Rockies. In the midcontinent and East, tectonic forces are squeezing the crust principally along an axis that runs essentially east-west. Yet the stress in Moodus seemed to point along a north-west-trending axis.

Studies in the new, deeper borehole apparently have resolved this conflict. They indicate that the stress in Moodus does indeed run in the normal east-west direction, says Tom Statton of Woodward-Clyde Consultants in Wayne, N.J., which supervised the borehole project.

As if one cue, the most recent earthquake swarm started in September of 1987, only a month after researchers ended the borehole stress experiments. While the quakes continued, Woodward-Clyde obtained funds to set up a network of eight seismometers, placed within a kilometer of the borehole. They found that during the earthquakes, subsurface rocks broke along north-south fractures. This pattern suggests that the stress producing the earthquakes pointed in the direction indicated by the borehole tests.

The network also told researchers the earthquake centers are quite concentrated. All the seismic energy emanated from a small plot of cruts--a sphere with a radius of one-quarter kilometer and centered at a depth of 1.5 km, says Statton.

Putting all this together, scientists can tell where the earthquakes are occurring and how the rocks are moving below ground. But one big problem remains: They can't link the rock movement to any known fault in the area.

By mapping the surface geology, scientists located the major faults that dominate this part of Connecticut. In fact, the borehole intersects one of these old, large structures, the Honey Hill fault, which is visible at the surface farther to the south. Yet this fault faces the wrong direction to account for the motions measured in Moodus, says David London, a geologist at the University of Oklahoma in Norman, who has mapped the region.

Other known faults also fail the test. "We have not identified any fault that we confidently feel is moving in the moderen earthquakes," Ebel says.

Part of the problem is that the fractures driving the Moodus earthquakes never reach the surface. In this respect, Moodus behaves as its geologic neighbors do. "In eastern North America, there has never been a documented case of an earthquake breaking the surface," says Shelton S. Alexander of Pennsylvania State University in University Park. This stands in contrast to the West, where faults, like the well-known San Andreas, often rupture the surface.

The fractures behind the Moodus earthquakes are small, probably a few hundred meters at most in length, Ebel says. They could be isolated ruptures on an older fault that has moved in the past, or they could be new breaks that do not fit